Mold, optical element and method for manufacturing the same

ABSTRACT

A method for manufacturing a mold or an optical element provided with a fine surface roughness according to an embodiment of the present invention is a method in which the fine surface roughness has been manufactured using a reactive etching apparatus. In the apparatus a substrate or a film made of a semiconductor or a metal which reacts with sulfur hexafluoride is placed, and into which a mixed gas of sulfur hexafluoride and oxygen is introduced and tuned into plasma such that oxides are made to be scattered on a surface of the substrate or the film, the surface of the substrate or the film is made to undergo etching by the sulfur hexafluoride while the oxides function as an etching mask, and thus the fine surface roughness is formed on the surface of the substrate or the film.

CROSS REFERENCE TO RELATED APPLICATION

This is a Continuation-in-Part of International Patent Application No.PCT/JP2013/061889 filed Apr. 23, 2013, which designates the U.S. and waspublished under PCT Article 21(2) in English, which claims priority fromU.S. Provisional Patent Application No. 61/727,284, dated Nov. 16, 2012.This Continuation-in-Part application also claims priority from U.S.Provisional Patent Application No. 61/968,629 filed on Mar. 21, 2014.The contents of these applications are hereby incorporated by reference.

BACKGROUND

1. Field

The present invention relates to a method for manufacturing a mold or anoptical element provided with a fine surface roughness, the mold and theoptical element.

2. Description of the Related Art

Anti-reflective structures having grating shapes, the pitch (or theperiod) of the grating being smaller than the wavelength of light, areused in optical elements. As a method for manufacturing molds for suchanti-reflective structures, a method in which a resist undergoespatterning by interference exposure or using an electron-beamlithography system and then etching or electroforming is carried out isknown (for example, WO2006/129514).

By the method in which an electron-beam lithography system is used, apattern with a fine pitch can be formed, and lithography or patternforming on curved surfaces can be realized. However, the requiredmanufacturing time excessively increases as an area in which pattern isto be formed increases. Accordingly, from a practical standpoint, themaximum area in which pattern can be formed is 10 mm square at most.

The method in which interference exposure is used carries an advantagethat a large area can be patterned at a time, but in the method theresolution is restricted. Thus, the pitch cannot be made highly fine.Further, when the method is applied to patterning on curved surfaces,degree of flexibility in design is low. Accordingly, there has been aproblem that anti-reflection property deteriorates in the lowerwavelength area of visible light.

Thus, methods in which patterning is used are complicated in process andare time-consuming.

On the other hand, a method for manufacturing a mold for ananti-reflective structure, which does not require patterning, has beendeveloped (for example, U.S. Pat. No. 8,187,481B1).

However, the method described in U.S. Pat. No. 8,187,481B1 has a problemwhen the method is applied to manufacture a high-performanceanti-reflective structure. Concerning the problem, description will begiven later in comparison with the present invention.

Further, black silicon for solar cells has been already developed.However, the technical field of black silicon and that of molds foroptical elements completely differ from each other. The both areirrelevant to each other, and there is nothing that suggests somerelationship between the both.

Recently, other methods for manufacturing anti-reflective structureswithout the necessity of patterning process of resist have beendeveloped. Among the methods, there are a method in which a fine surfaceroughness is formed by coating nanoparticles on a surface of a substrate(for example, JP2012-40878) and a method in which a fine surfaceroughness is formed using anodic oxidation porous alumina as a mold (forexample, JP2014-51710). These methods are supposed to be applied to asurface having a large area or a curved surface. However, because of theproperties of the manufacturing methods, pitch of the surface roughnessis restricted to 1 micrometer or less. Accordingly, the surfaceroughness can hardly be applied to optical elements functioning withinfrared rays, for example.

Thus, a method for manufacturing a mold or an optical element by whichfine surface roughness having pitches of a wide range including infraredregion can be formed on a surface having a large area or on a curvedsurface, has not been developed.

Accordingly, there is a need for a method for manufacturing a mold or anoptical element by which fine surface roughness having pitches of a widerange including infrared region can be formed on a surface having alarge area or on a curved surface.

SUMMARY

A method for manufacturing a mold or an optical element provided with afine surface roughness according to an embodiment of the presentinvention is a method in which the fine surface roughness has beenmanufactured using a reactive etching apparatus. In the apparatus asubstrate or a film made of a semiconductor or a metal which reacts withsulfur hexafluoride is placed, and into which a mixed gas of sulfurhexafluoride and oxygen is introduced and tuned into plasma such thatoxides are made to be scattered on a surface of the substrate or thefilm, the surface of the substrate or the film is made to undergoetching by the sulfur hexafluoride while the oxides function as anetching mask, and thus the fine surface roughness is formed on thesurface of the substrate or the film.

According to the present embodiment, a mold or an optical elementprovided with a fine surface roughness can be manufactured by asimplified process without the necessity of patterning process for anetching mask. Further, a mold or an optical element provided with a finesurface roughness having a pitch in a wide range including a visiblelight region and an infrared region can be obtained.

A mold according to another embodiment of the present invention, ismanufactured by the method described in the above-described firstembodiment.

In a mold according to another embodiment of the present invention, thefine surface roughness on the mold is used to form an anti-reflectivestructure.

In a mold according to another embodiment of the present invention, thefine surface roughness on the mold is used to form a diffusingstructure.

An optical element according to another embodiment of the presentinvention, is manufactured by the method described in theabove-described first embodiment.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a construction of a reactive ion etching apparatus used fora method for manufacturing a mold or an optical element having a surfaceroughness;

FIG. 2 is a flowchart for illustrating the principle of a method formanufacturing a mold for anti-reflective structure according to thepresent invention;

FIGS. 3A and 3B illustrate a method for manufacturing a mold having afine surface roughness on a flat surface;

FIGS. 4A and 4B illustrate a method for manufacturing an optical elementhaving a fine surface roughness;

FIG. 5 is a flowchart for determining etching conditions of a method formanufacturing a mold for anti-reflective structure as an example of themethod for manufacturing a mold according to the present invention;

FIG. 6 shows a relationship between etching time and pitch of the finesurface roughness in the case that the power of the high frequency powersupply is set to 100 watts and the etching conditions shown in Table 1are maintained and a relationship between etching time and pitch of thefine surface roughness in the case that the power of the high frequencypower supply is set to 200 watts and the etching conditions shown inTable 1 are maintained;

FIG. 7 shows a relationship between etching time and depth of the finesurface roughness in the case that the power of the high frequency powersupply is set to 100 watts and the etching conditions shown in Table 1are maintained and a relationship between etching time and depth of thefine surface roughness in the case that the power of the high frequencypower supply is set to 200 watts and the etching conditions shown inTable 1 are maintained;

FIG. 8 shows relationships between wavelength and transmittance ofinfrared rays which enter substrates having different types of surfaceroughness;

FIG. 9 illustrates transmittance;

FIG. 10 shows a relationship between wavelength of light transmittanceof which is to be increased and pitch of fine surface roughness forincreasing the transmittance;

FIG. 11 shows a photo of the substrate 1 without a fine surfaceroughness, the substrate 2 with a fine surface roughness for visiblelight, and the substrate with the fine surface roughness 3;

FIG. 12 shows a scanning electron microscope photo of the fine surfaceroughness 3;

FIG. 13 is a flowchart for determining etching conditions of the amethod for manufacturing a mold for anti-reflective structure accordingto the present invention;

FIGS. 14A and 14B illustrate how a mold for anti-reflective structure isformed on a flat surface;

FIGS. 15A, 15B and 15C illustrate how a mold for anti-reflectivestructure is formed on a curved surface;

FIG. 16 is a flowchart for illustrating a method for manufacturing amold for a diffraction grating provided with an anti-reflectivestructure;

FIGS. 17A, 17B and 17C are diagrams for illustrating a method formanufacturing a mold for a diffraction grating provided with ananti-reflective structure;

FIGS. 18A, 18B and 18C are diagrams for illustrating patterning of anetching mask;

FIG. 19 shows a scanning electron microscope photo of a mold foranti-reflective structure manufactured by a method according to thepresent invention;

FIG. 20 shows a scanning electron microscope photo of a mold for adiffraction grating provided with an anti-reflective structure; and

FIG. 21 shows relationships between reflectance and wavelength of asurface provided with an anti-reflective structure manufactured by themethod according to the present invention, a surface provided with ananti-reflective structure manufactured by a method according to a priorart and a surface without an anti-reflective structure.

DETAILED DESCRIPTION

FIG. 1 shows a construction of a reactive ion etching apparatus 200 usedfor manufacturing a mold or an optical element having a surfaceroughness. The reactive ion etching apparatus 200 has an etching chamber201. Gases are fed to the evacuated etching chamber 201 through a gasfeed port 207. The etching chamber 201 is further provided with a gasexhaust port 209 to which a valve 217 is attached. The gas pressure inthe etching chamber 201 can be controlled to a desired value by acontroller 215 which is configured to manipulate the valve 217 accordingto a measurement of a gas pressure gauge 213 installed at the etchingchamber 201. An upper electrode 203 and a lower electrode 205 areprovided in the etching chamber 201. Plasma can be generated by applyinghigh frequency voltage between the both electrodes using high frequencypower supply 211. On the lower electrode 205 a substrate 101 which is abase material of a mold is placed. The lower electrode 205 can be cooledat a desired temperature by a cooling device 219. The cooling device 219may be a water-cooling chiller, for example. The reason that the lowerelectrode 205 is cooled is to control the etching reaction by keepingthe temperature of the substrate 101 at a desired value.

The gas to be fed to the etching chamber 201 is a mixture of sulfurhexafluoride gas and oxygen gas. The material of the substrate is asemiconductor or a metal which reacts with sulfur hexafluoride.

FIG. 2 is a flowchart for illustrating the principle of a method formanufacturing a mold for anti-reflective structure according to anembodiment of the present invention.

In step S1010 of FIG. 2, a high frequency voltage is applied to themixture of gases such that it is turned into plasma to carry out plasmadry etching.

In step S1020 of FIG. 2, oxygen ions in the plasma bind to ions of themetal or the semiconductor of the substrate, which have reacted with thefluorine-containing gas (sulfur hexafluoride gas), resultant oxidesdeposit at random positions on the surface of the substrate. Theabove-described oxides are hardy etched by sulfur hexafluoride, andtherefore function as an etching mask.

In step S1030 of FIG. 2, portions on the surface of the substrate, whichare not covered with the oxides undergo etching while the oxidesfunction as a mask. As a result, a surface roughness is formed on thesurface of the substrate.

As described above, the used gas is a mixture of sulfur hexafluoride(SF₆) gas and oxygen gas.

The material of the substrate is a semiconductor or a metal which reactswith sulfur hexafluoride. More specifically, the material is silicon,titanium, tungsten, tantalum, a titanium alloy which is made by addingother elements to titanium, a tungsten alloy which is made by addingother elements to tungsten, or the like.

FIGS. 3A and 3B illustrate a method for manufacturing a mold having afine surface roughness on a flat surface.

FIG. 3A shows a cross section of a substrate 101 to which etching hasnot been carried out.

FIG. 3B shows a cross section of the substrate 101 which is providedwith a fine surface roughness. The fine surface roughness has beenformed by etching carried out using the reactive ion etching apparatus.In FIG. 3B, the size of the fine surface roughness is displayed in anenlarged view in comparison with the substrate for the sake of easierunderstanding.

FIGS. 4A and 4B illustrate a method for manufacturing an optical elementhaving a fine surface roughness.

FIG. 4A shows a cross section of an optical element made of silicon. Theoptical element has a curved surface, which has been shaped by cuttingor the like. The optical element made of silicon is used for infraredrays.

FIG. 4B shows a cross section of the optical element made of silicon,which is provided with a fine surface roughness. The fine surfaceroughness has been formed by etching carried out using the reactive ionetching apparatus. The fine surface roughness of the optical elementfunctions as an anti-reflective structure. In FIG. 4B, the size of thefine surface roughness is displayed in an enlarged view in comparisonwith the optical element for the sake of easier understanding.

FIG. 5 is a flowchart for determining etching conditions of a method formanufacturing a mold for anti-reflective structure as an example of themanufacturing method according to the present invention.

In step S2010 in FIG. 5, initial values of the etching conditions areselected.

In step S2020 in FIG. 5, etching is carried out on the substrate underthe selected etching conditions using the reactive ion etchingapparatus.

In step S2030 in FIG. 5, a reflectance of the manufactured mold isevaluated.

In step S2040 in FIG. 5, a shape of the manufactured mold is evaluated.The shape is evaluated using a scanning electron microscope, forexample.

In step S2050 in FIG. 5, it is determined whether or not themanufactured mold is appropriate for a mold for anti-reflectivestructure. If the manufactured mold is appropriate, the process isterminated. If the manufactured mold is not appropriate, the processgoes to step S2060.

In step S2060 in FIG. 5, the etching conditions are adjusted.

The etching conditions will be described in detail below.

Table 1 shows some of the etching conditions.

TABLE 1 Operation Mixture ratio of Cooling pressure SF₆ and O₂temperature 1 Pa 50 mL/min:50 mL/min 3° C.

Into the etching chamber 201 of the reactive etching apparatus 200, themixed gas of sulfur hexafluoride and oxygen is fed. An amount of feed ofsulfur hexafluoride and that of oxygen are 50 milliliters per minuterespectively. The pressure in the etching chamber 201 is controlled at 1pascal. The temperature of the lower electrode 205 on which thesubstrate 101 is set is controlled at 3 degrees centigrade. Thesubstrate 101 is made of silicon.

FIG. 6 shows a relationship between etching time and pitch of the finesurface roughness in the case that the power of the high frequency powersupply 211 is set to 100 watts and the etching conditions shown in Table1 are maintained, and a relationship between etching time and pitch ofthe fine surface roughness in the case that the power of the highfrequency power supply 211 is set to 200 watts and the etchingconditions shown in Table 1 are maintained. The horizontal axis in FIG.6 represents etching time while the vertical axis in FIG. 6 representspitch of the fine surface roughness. The unit of time is minute and theunit of pitch is micrometer. The frequency of the high frequency powersupply 211 is 13.56 MHz.

Pitch of the fine surface roughness is an average of distance in thedirection parallel to the substrate surface between adjacent convexportions or between adjacent concave portions in a cross section of thefine surface roughness. The view of the cross section can be obtained byan atomic force microscope or the like. The pitch can be obtainedthrough Fourier analysis of the cross section shape of the fine surfaceroughness.

According to FIG. 6, pitch of the fine surface roughness increases withetching time. Further, rate of increase of pitch against time increaseswith power of the high frequency power supply 211.

FIG. 7 shows a relationship between etching time and depth of the finesurface roughness in the case that the power of the high frequency powersupply 211 is set to 100 watts and the etching conditions shown in Table1 are maintained, and a relationship between etching time and depth ofthe fine surface roughness in the case that the power of the highfrequency power supply 211 is set to 200 watts and the etchingconditions shown in Table 1 are maintained. The horizontal axis in FIG.7 represents etching time while the vertical axis in FIG. 7 representsdepth of the fine surface roughness. The unit of time is minute and theunit of depth is micrometer.

Depth of the fine surface roughness is an average of distance in thedirection perpendicular to the substrate surface between adjacent convexand concave portions in a cross section of the fine surface roughness.

According to FIG. 7, depth of the fine surface roughness increases withetching time. Further, rate of increase of depth against time increaseswith power of the high frequency power 211.

As described above, by adjusting the etching conditions including powerof the high frequency power supply 211 and etching time, fine surfaceroughness having pitches and depths which correspond to visible lightregion and infrared region can be manufactured.

FIG. 8 shows relationships between wavelength and transmittance ofinfrared rays which enter substrates having different types of surfaceroughness. The horizontal axis in FIG. 8 represents wavelength of theinfrared rays which enter the substrates while the vertical axis in FIG.8 represents transmittance of the infrared rays. In FIG. 8 the solidline represents the relationship between wavelength and transmittance ofinfrared rays which enter a substrate without fine surface roughness. InFIG. 8 the two-dotted line represents the relationship betweenwavelength and transmittance of infrared rays which enter the substratehaving the fine surface roughness which has been manufactured under theetching conditions 1 which will be described below. In FIG. 8 the dashedline represents the relationship between wavelength and transmittance ofinfrared rays which enter the substrate having the fine surfaceroughness which has been manufactured under the etching conditions 2which will be described below.

FIG. 9 illustrates transmittance. Transmittance is a ratio of an amountof transmitted light to an amount of the incident light. Thetransmittance changes by the function of the fine surface roughness 1011of the substrate 101.

Table 2 shows the etching conditions 1 and the etching conditions 2.

TABLE 2 Etching Operation Mixture ratio of Cooling conditions pressureSF₆ and O₂ Power Time temperature 1 1 Pa 50 mL/min:50 mL/min 100 W 120minutes 3° C. 2 1 Pa 50 mL/min:50 mL/min 200 W 120 minutes 3° C.

The fine surface roughness which has been manufactured under the etchingconditions 1 will be hereinafter referred to as the fine surfaceroughness 1. The pitch of the fine surface roughness 1 is 1.0 micrometerwhile the depth of the fine surface roughness 1 is 1.21 micrometers. Theratio of pitch to depth of the fine surface roughness 1 is 0.83. Thefine surface roughness which has been manufactured under the etchingconditions 2 will be hereinafter referred to as the fine surfaceroughness 2. The pitch of the fine surface roughness 2 is 3.0 micrometerwhile the depth of the fine surface roughness 2 is 2.79 micrometers. Theratio of pitch to depth of the fine surface roughness 2 is 1.1.

According to FIG. 8, transmittance of the substrate having the finesurface roughness 1 is higher than that of the substrate without finesurface roughness in the wavelength range from 2 to 15 micrometers.Particularly, in the wavelength range from 3 to 7 micrometers,transmittance of the substrate having the fine surface roughness 1 ishigher than that of the substrate without fine surface roughness by 10%or more. Transmittance of the substrate having the fine surfaceroughness 2 is higher than that of the substrate without fine surfaceroughness in the wavelength range from 6 to 15 micrometers.Particularly, in the wavelength range from 7 to 12 micrometers,transmittance of the substrate having the fine surface roughness 2 ishigher than that of the substrate without fine surface roughness by 7%or more. From the above, the pitch of fine surface roughness forincreasing transmittance, that is, reducing reflectance should be fromone fifth (⅕) to one half (½) of the wavelength of light transmittanceof which is to be increased.

FIG. 10 shows an example of a relationship between wavelength of lighttransmittance of which is to be increased and pitch of fine surfaceroughness for increasing the transmittance. The horizontal axis in FIG.10 represents wavelength of light transmittance of which is to beincreased while the vertical axis in FIG. 10 represents pitch of finesurface roughness for increasing the transmittance.

Fine surface roughness having a pitch which is larger than that of thefine surface roughness 2 was manufactured. The fine surface roughnesswill be hereinafter referred to as fine surface roughness 3.

Table 3 shows the etching conditions for the fine surface roughness 3.

TABLE 3 Operation Mixture ratio of Cooling pressure SF₆ and O₂ PowerTime temperature 1 Pa 50 mL/min:40 mL/min 300 W 120 minutes 3° C.

The pitch of the fine surface roughness 3 is 18.0 micrometers while thedepth of the fine surface roughness 3 is 6.0 micrometers. The ratio ofpitch to depth of the fine surface roughness 3 is 3.0

In the etching conditions shown in Table 3, the amount of feed of oxygenis smaller than that of sulfur hexafluoride. As a result, distancesbetween oxides which are deposited on the substrate surface and functionas an etching mask become greater. Accordingly, the ratio of pitch todepth of the fine surface roughness 3 becomes greater than those of thefine surface roughness 1 and the fine surface roughness 2. As describedabove, by changing the ratio of the amount of feed of sulfurhexafluoride and the amount of feed of oxygen, the ratio of pitch todepth of the fine surface roughness can be changed.

FIG. 11 shows a photo of the substrate 1 without a fine surfaceroughness, the substrate 2 with a fine surface roughness for visiblelight, and the substrate with the fine surface roughness 3. The pitch ofthe fine surface roughness of the substrate 2 is 0.2 micrometers.Reflection on the surface of the substrate 2 is reduced by the finesurface roughness, and therefore the substrate 2 looks darker than thesubstrate 1. The pitch of the fine surface roughness 3 is much greaterthan wavelengths of the visible light. On the other hand, values ofdistance in the direction parallel to the substrate surface betweenadjacent convex portions or between adjacent concave portions are notconstant and are distributed in a predetermined range. Accordingly, thefine surface roughness 3 of the substrate 3 causes diffracted lights ofvarious orders of diffraction and of various wavelengths, and thus thesubstrate 3 looks more whitish than the substrate 1. This means that thesubstrate 3 with the fine surface roughness 3 causes diffusion of thevisible light.

Thus, the substrate 3 with the fine surface roughness 3 functions as adiffuser plate. Thus, a mold for a diffusing structure is obtained.

FIG. 12 shows a scanning electron microscope photo of the fine surfaceroughness 3.

A method for manufacturing a mold for anti-reflective structureaccording to the present invention will be described using anotherexample. A silicon wafer is used as the substrate.

Table 4 shows characteristics of the silicon wafer used in the example.

TABLE 4 Type of electric conductivity N or P Dopant Phosphorus or BoronCrystallographic axis (100) ± 1.0° Resistivity 1.0-10.0 Ω · cm

Table 5 shows etching conditions in the example.

TABLE 5 High frequency Gas Feed of Feed of power Cooling Reactionpressure SF6 oxygen (RF power) temperature time 2 Pa 50 ml/min 50 ml/min200 W 5° C. 20 minutes

The frequency of the high frequency power is 13.56 MHz and the voltageis 200 V.

The pitch of the fine structure of the mold for anti-reflectivestructure manufactured by the above-described manufacturing conditionsis approximately 0.2 micrometers while the depth thereof isapproximately 0.3 micrometers. The aspect ratio is approximately 1.5.

FIG. 13 is a flowchart for determining etching conditions of the methodfor manufacturing a mold for anti-reflective structure according to thepresent invention.

In step S3010 in FIG. 13, initial values of the etching conditions areselected. More specifically, for example, the values shown in Table 5are selected.

In step S3020 in FIG. 13, etching is carried out on the substrate underthe selected etching conditions using the reactive ion etchingapparatus.

In step S3030 in FIG. 13, a reflectance of the manufactured mold isevaluated.

In step S3040 in FIG. 13, a shape of the manufactured mold is evaluated.The shape is evaluated using a scanning electron microscope, forexample.

In step S3050 in FIG. 13, it is determined whether or not themanufactured mold is appropriate for a mold for anti-reflectivestructure. If the manufactured mold is appropriate, the process isterminated. If the manufactured mold is not appropriate, the processgoes to step S3060.

In step S3060 in FIG. 13, the etching conditions are adjusted. How toadjust the etching conditions will be described below.

The aspect ratio of the fine structure should be 0.8 or more. In orderto change the aspect ratio, a ratio of partial pressures of the gasses,the cooling temperature of the substrate, and etching time are mainlyadjusted. When the partial pressure of SF₆ gas in the mixed gas israised, the etching rate becomes higher. When the cooling temperature ofthe substrate is lowered, the reaction for generating silicon oxide(SiO) is promoted, and therefore formation of coated portions forpreventing etching (the mask) is promoted. Accordingly, when the etchingtime (reaction time) is increased under the above-described conditions,the aspect ratio becomes greater.

The pitch of the fine structure should be 0.35 micrometers or less suchthat the pitch is smaller than wavelengths of the visible light. Inorder to change the pitch of the fine structure, a ratio between partialpressures of the gasses and the cooling temperature of the substrate areadjusted. When the ratio of partial pressure of oxygen is raised and thecooling temperature of the substrate is lowered, the pitch of the finestructure becomes smaller.

Functions of various parameters can be summarized as below.

When the ratio of partial pressure of sulfur hexafluoride (SF₆) in themixed gas is raised, the etching rate becomes higher.

When the cooling temperature of the substrate is lowered, the reactionfor generating silicon oxide (SiO) is promoted, and therefore formationof coated portions for preventing etching (the mask) is promoted.

When reaction time is increased, etching is promoted.

When the gas pressure of the mixed gas is raised, the etching ratebecomes higher.

When the power of the high frequency power supply is raised, the etchingrate becomes higher.

However, when the ratio of partial pressure of sulfur hexafluoride (SF₆)in the mixed gas is too high, silicon oxide (SiO) is not generated, andtherefore coated portions for preventing etching (the mask) are notformed. Accordingly, the grating like structure is not formed. Further,when the ratio of partial pressure of oxygen in the mixed gas is toohigh, or the cooling temperature of the substrate is too low, coatedportions for preventing etching (the mask) are excessively generated,and therefore etching is not carried out. Accordingly, the grating likestructure is not formed.

Accordingly, the above-described various parameters should be adjustedin predetermined ranges.

Table 6 shows the ranges of adjustment of the various parameters in theabove-described case (in which the material of the substrate is siliconand the mixed gas includes sulfur hexafluoride (SF₆) and oxygen).

TABLE 6 High frequency Gas Ratio of oxygen power Cooling Reactionpressure in mixed gas (RF power) temperature time 1-5 Pa 30-70% 50-5000W 30° C. or less 5-300 minutes

Table 7 shows the ranges of adjustment of the various parameters in thecase in which the material of the substrate is one of titanium,tungsten, tantalum, a titanium alloy which is made by adding otherelements to titanium, and a tungsten alloy which is made by adding otherelements to tungsten silicon and the mixed gas includes sulfurhexafluoride (SF₆) and oxygen.

TABLE 7 High frequency Gas Ratio of oxygen power Cooling Reactionpressure in mixed gas (RF power) temperature time 1-5 Pa 30-70% 50-5000W 30° C. or less 5-600 minutes

An advantage of the case in which silicon is used as the material of thesubstrate is that machining is easier, while an advantage of the case inwhich a metal is used as the material of the substrate is that the moldis superior in durability.

In the above-described embodiment, the mixed gas of sulfur hexafluorideand oxygen is used. In place of sulfur hexafluoride, otherfluorine-containing gases (carbon tetrafluoride, trifluoromethane andthe like) can also be used.

FIGS. 14A and 14B illustrate how a mold for anti-reflective structure isformed on a flat surface.

FIG. 14A shows a cross section of the substrate 1101 to which etchinghas not been carried out.

FIG. 14B shows a cross section of the substrate 1101 which is providedwith a shape of anti-reflective structure on a surface. The shape ofanti-reflective structure has been formed through etching which has beencarried out using the reactive ion etching apparatus.

FIGS. 15A, 15B and 15C illustrate how a mold for anti-reflectivestructure is formed on a curved surface.

FIG. 15A shows a cross section of a mold core 1110 which is providedwith a curved surface. The curved surface is formed by cutting, forexample.

FIG. 15B shows a cross section of a mold core 1110 which is providedwith a thin film 111 of base material formed on a surface of the core.The thin film 111 of base material is formed by sputtering, vapordeposition or the like.

FIG. 15C shows a cross section of a mold core which is obtained byforming a shape of anti-reflective structure on a surface of the thinfilm 1111 of the substrate of what is shown in FIG. 15B. The shape ofanti-reflective structure has been formed through etching which has beencarried out using the reactive ion etching apparatus. According to themethod illustrated by FIGS. 15A, 15B and 15C, a mold for anti-reflectivestructure can be manufactured on any curved surfaces.

FIG. 16 is a flowchart for illustrating a method for manufacturing amold for a diffraction grating provided with an anti-reflective finestructure.

FIGS. 17A, 17B and 17C are diagrams for illustrating the method formanufacturing a mold for a diffraction grating provided with ananti-reflective fine structure.

In step S4010 in FIG. 16, a shape of anti-reflective structure is formedon a surface of a substrate 1121 through etching which is carried outusing the reactive ion etching apparatus.

FIG. 17A shows a cross section of the substrate 1121 which has undergoneetching.

In step S4020 in FIG. 16, on the surface of the substrate 1121, on whichthe shape of anti-reflective structure has been formed through etchingwhich has been carried out using the reactive ion etching apparatus, apatterning of etching mask for a diffraction grating is carried out.

FIG. 17B shows a cross section of the substrate 1121, on a surface ofwhich the patterning of an etching mask 1125 for the diffraction gratinghas been carried out. The patterning of the etching mask 1125 will bedescribed later.

In step S4030 in FIG. 16, the substrate 1121, on a surface of which thepatterning of the etching mask 1125 for the diffraction grating has beencarried out is further made to undergo etching using the reactive ionetching apparatus.

In step S4040 in FIG. 16, the etching mask 1125 is removed. The removalof the etching mask 1125 will be described later.

FIG. 17C shows a cross section of a mold for a diffraction gratingprovided with an anti-reflective fine structure. The mold ismanufactured by the method illustrated in the flowchart of FIG. 16.

FIGS. 18A, 18B and 18C are diagrams for illustrating patterning of anetching mask.

FIG. 18A shows a cross section of a substrate 1121, on a surface ofwhich patterning of a resist 1123 for a diffraction grating has beencarried out.

FIG. 18B shows a cross section of the substrate 1121, on a surface ofwhich patterning of the resist 1123 for the diffraction grating has beencarried out and then a metal 1125 which is resistant to reaction withfluorine-containing gases, such as chromium and nickel has beendeposited.

FIG. 18C shows a cross section of the substrate 121, on a surface ofwhich patterning of the resist 1123 for the diffraction grating has beencarried out, the metal 1125 which is resistant to reaction withfluorine-containing gases, such as chromium and nickel has beendeposited and then the resist 1123 has been removed. The metal 1125 suchas chromium and nickel, shown in FIG. 18C functions as the etching mask.

The resist 1123 shown in FIG. 18A can also be used as the etching mask.However, etch selectivity (a difference in etching rate) of thesubstrate against the resist is smaller than etch selectivity of thesubstrate against a metal such as chromium and nickel, and thereforedepth of etching is smaller.

FIG. 19 shows a scanning electron microscope photo of a mold foranti-reflective structure manufactured by a method according to thepresent invention. The pitch of the fine structure of theanti-reflective structure is approximately 0.2 micrometers.

FIG. 20 shows a scanning electron microscope photo of a mold for adiffraction grating provided with an anti-reflective fine structure. Thepitch of the diffraction grating is approximately 2 micrometers whilethe pitch of the fine structure of the anti-reflective structure isapproximately 0.2 micrometers.

FIG. 21 shows relationships between reflectance and wavelength of asurface provided with an anti-reflective structure manufactured by themethod according to the present invention, a surface provided with ananti-reflective structure manufactured by a method according to a priorart (a method using an electron beam lithography system) and a surfacewithout an anti-reflective structure. The horizontal axis of FIG. 21represents wavelength while the vertical axis of FIG. 21 representsreflectance. Reflectance of the surface provided with theanti-reflective structure manufactured by the method according to thepresent invention is smaller over the whole range of wavelength than thereflectance of the surface provided with the anti-reflective structuremanufactured by the method according to a prior art. This demonstratesthat an anti-reflective structure having higher performance can bemanufactured according to the present invention.

By the method for manufacturing a mold for an anti-reflective structureaccording to the present invention, an anti-reflective structure havinghigher performance can be manufactured without the use of patterning.According to the present method, a mold for an anti-reflective structureof a large area can be manufactured without any other constrains thanthe area of the reactive etching apparatus. Further, according to thepresent method, a mold for an anti-reflective structure for molding ananti-reflective fine structure on any curved surface and a mold for ananti-reflective structure for molding a diffraction grating providedwith an anti-reflective fine structure can be manufactured.

According to the present invention, a mold for an anti-reflectivestructure used for visible light and infrared rays, an optical elementprovided with an anti-reflective structure, and a mold for diffusingstructure can be obtained without the necessity of patterning.

1. A method for manufacturing a mold or an optical element provided witha fine surface roughness, wherein the fine surface roughness has beenmanufactured using a reactive etching apparatus, in which a substrate ora film made of a semiconductor or a metal which reacts with sulfurhexafluoride is placed, and into which a mixed gas of sulfurhexafluoride and oxygen is introduced and tuned into plasma such thatoxides are made to be scattered on a surface of the substrate or thefilm, the surface of the substrate or the film is made to undergoetching by the sulfur hexafluoride while the oxides function as anetching mask, and thus the fine surface roughness is formed on thesurface of the substrate or the film.
 2. A mold manufactured by themethod according to claim
 1. 3. A mold according to claim 2, wherein thefine surface roughness on the mold is used to form an anti-reflectivestructure.
 4. A mold according to claim 2, wherein the fine surfaceroughness on the mold is used to form a diffusing structure.
 5. Anoptical element manufactured by the method according to claim 1.